1. Field of the Invention
The present invention relates to a dielectric filter, a dielectric duplexer, and a communication apparatus using the same, which are used, for instance, in a microwave band.
2. Description of the Related Art
FIGS. 11A and 11B show an example of a structure of a conventional dielectric filter, in which the effects of electromagnetic field leakage are reduced by providing an open terminal indented from an end surface of a dielectric block, thereby obtaining the desired coupling between resonators. FIG. 11A is a perspective view from the open end, and FIG. 11B is a cross-sectional view taken horizontally through the center.
The dielectric filter comprises inner conductors 13a and 13b, which are provided in through-holes 12a and 12b, respectively, running between two opposing end surfaces of a dielectric block 11. A ground conductor 15 is provided substantially over all outer surfaces of the dielectric block 11. Input/output electrodes 16a and 16b are provided extending from one major surface, which constitutes a mounting surface, to the side faces. In the inner conductors 13a and 13b (hereinafter referred to as open-side end surface) of each of the through-holes 12a and 12b, portions devoid of inner conductor i.e., nonconductive portion g are provided near one open surface. These nonconductive portions g function as open terminals, and the ground conductor 15 is short-circuited at the other open surface (hereinafter referred to as short-circuiting-side end surface).
The dielectric filter comprises two resonators formed by the inner conductors 13a and 13b. The resonators are coupled together by stray capacitance Cs, created in the nonconductive portions g, and are coupled externally by external coupling capacitance Ce, created between the inner conductors 13a and 13b and the corresponding inpu/output electrodes 16a and 16b. The external coupling capacitance Ce is adjusted by altering the position and area of the input/output electrodes 16a and 16b to achieve an appropriate external coupling.
Ordinarily, the greater the area of the input/output electrodes, and the closer the electrodes are positioned to the short-circuiting-side end surface, the worse the Qo of the resonators. That is, in order to prevent deterioration in the Qo, the input/output electrodes must be positioned closer to the open-side end surface and their areas must be reduced.
However, the conventional dielectric filter described above has a disadvantage in that, although it is possible to prevent deterioration in the Qo by providing the input/output electrodes near to the open-side end surface, the nonconductive portions g are provided at positions indented from the open-side end surface, and consequently the opposing area of the input/output electrodes and the inner conductors is reduced, thereby reducing the external coupling capacitance and making it impossible to obtain a desired external coupling.
There is a further problem in that operations such as changing the orientation of the dielectric block are necessary to provide the input/output electrodes on opposite side faces as in the conventional example described above, consequently increasing the cost of providing the input/output electrodes. Furthermore, when the input/output electrodes are provided only on the mounting surface, it is not easy to confirm the state of the solder after mounting.